1. Field of the Invention
The present invention relates to the field of color electrography. More particularly, it relates to an apparatus and method for reducing the grainy appearance of electrographically-produced color prints.
2. Description of Prior Art
The commonly assigned U.S. Pat. No. 4,994,827, entitled MULTICOLOR IMAGING METHOD AND APPARATUS; U.S. Pat. No. 5,047,791, entitled ELECTROSTATIC COLOR PRINTING APPARATUS and U.S. Pat. No. 5,151,717, entitled IMAGING METHOD AND APPARATUS USING DISCRETE SHEETS, all issued to Feraydoon S. Jamzadeh et al, disclose electrophotographic printing apparatus for producing full color prints of photographic quality. Such an apparatus comprises an electronic scanner for scanning the individual frames of a color negative film to convert the color image information to electronic form. In scanning a single frame of plural colors, three or four video signals are produced, each representing the color image information in a different portion of the spectrum (e.g., red, green and blue). Each of such video signals is stored in memory and, prior to being used to control the intensity of a laser scanner used to record color-separated image information on a photoconductive recording element, is electronically processed to effect, for example, edge enhancement "sharpening" or "crispening" of the image information. Various edge-enhancing techniques are well known; see, e.g., "Digital Image Processing" by William K. Pratt, published by John Wiley & Sons (1978).
In the above color printing process, a series of electrostatic images are sequentially recorded on the photoconductive recording element, each image representing color-separated image information. These electrostatic images are then developed with toner particles of different color (e.g., cyan, magenta, yellow and sometimes black), and the resulting toner images are transferred, one after the other, to a thermoplastic-coated paper receiver sheet. The edge enhancement, referred to above, has the effect of boosting the image density at the edges of all image patterns, the effect being a much sharper and crisper looking print.
In the above-mentioned printing process, each successive transfer of toner images becomes increasingly more difficult with each color-separated image. It will be appreciated that after the first color-separated toner image has been transferred to the thermoplastic surface of the receiver sheet, as applicable to electrostatic transfers, the untoned surface area on the receiver is smaller for the second toner image than it was for the first. Of course, after the second toner image has been transferred, there is even less untoned surface area on the receiver for the third toner image than there was for the second image. Since the toner particles comprise a thermoplastic material (see U.S. Pat. No. 4,968,578 for further description of how heating the receiver sheet to a degree sufficient to soften the toner is practiced) having a melting point substantially higher than that of the receiver's thermoplastic coating, toner will more readily transfer to the receiver's thermoplastic surface than to the surface of the previously deposited toner particles. Thus, the transfer efficiency of the second toner image is somewhat less than that of the first image, and the transfer efficiency of the third toner image is somewhat less than that of the second, and so on. As the transfer efficiency declines, the graininess of the image increases. Applying edge enhancement to a grainy image only acts to amplify the graininess of the image.